ABSTRACT The overall objective of this proposed research is to develop methods for the clinical assessment of lesion structure and activity. New imaging technologies employing non- ionizing radiation are needed to aid in caries diagnosis and management. It is not sufficient to simply detect caries lesions, methods are needed to assess the lesion activity and determine if chemical intervention is needed. Measurements of lesion structure and the optical and dimensional changes to lesions that occur during dehydration offer the potential for lesion activity assessment during a single examination. The central hypothesis underlying this proposal is that there are structural differences between active and arrested lesions and these differences can be quantified using near- IR (NIR) imaging methods. The objectives of this proposal will be achieved through the following specific aims: (1) To test the hypothesis that NIR reflectance measurements during lesion dehydration are more sensitive to changes in lesion structure and activity than existing methods and that NIR imaging can be used to assess the activity of coronal caries lesions in vivo. (2) To test the hypothesis that cross-polarization OCT (CP-OCT) can be used to assess changes in lesion structure and activity in the pits and fissures of primary teeth before and after chemical intervention. (3) To test the hypothesis that CP-OCT can be used to assess the activity of lesions on root surfaces and monitor the efficacy of conservative intervention in vivo. Success of this proposal will provide methods for assessing the activity of lesions in a single examination and for monitoring the efficacy of chemical intervention, thereby enabling the use of new intervention therapies to avert cavitation and restoration.